Seasonal, Fresh Water, and Timber Freeboard Marks
After the application of all the corrections to the basic Minimum Summer Freeboard are made, the result is the Minimum Summer Freeboard in salt water which will be assigned to the vessel. This freeboard may in no case be less than 50 mm. For ships having hatchways with covers other than steel or equivalent material on the freeboard deck or on superstructure decks situated forward of a point located a quarter of the ship's length from the forward perpendicular, the freeboard is not less than 150 mm.
Load Line Format
The freeboard is measured from the top of the deck amidships to the top of the line through the center of the load-line ring. Forward of the ring is a grid composed of lines indicating the maximum loadings in fresh water and for the different seasons, including the summer line which will be at the same level as the center of the ring.
Zones and Seasons
The regulations require a vessel to be so loaded when departing upon a voyage that at no time during any portion of the voyage will the applicable seasonal mark be submerged. The oceans of the world are divided under the regulations into various zones and seasonal areas according to the probable severity of the weather. In certain areas and times of the year where during the winter more severe weather may be expected the vessel is required not to load as deeply as is permitted in the summer.
These areas are the seasonal winter zones, and the times of the year when the winter mark is applicable are shown on a map attached to the regulations. During other times of the year, the summer mark at the center of the ring is applicable. Other zones, roughly corresponding to the temperate zone in the northern and southern hemispheres, are permanent summer zones under the regulations where the summer mark is applicable the year around.
Another zone, on either side of the equator, is a permanent tropical zone where the anticipated weather is generally less severe than might be expected in the summer in the temperate zone. In these zones vessels are permitted to load somewhat deeper any time during the year. Other areas between the permanent summer and tropical zones arc considered one or the other at specified different times of the year.
Calculation of Seasonal Marks
The various seasonal and fresh water marks are obtained as follows:
Tropical Freeboard Mark (T)
The line to mark the maximum loading in the tropical zone is obtained as a deduction from the Summer Freeboard of one forty-eighth of the summer draft measured from the top of the keel to the center of the ring. Again, the freeboard must not be less than 50 mm or 150 mm.
Winter Freeboard Mark (W)
The line to mark the maximum loading in winter zones is obtained by an addition to the Summer Freeboard of one forty-eighth of the molded summer draft.
Winter North Atlantic Freeboard (WNA)
The minimum freeboard for ships of not more than 100 m in length which enter any part of the North Atlantic defined in the regulations during the winter seasonal period shall be the winter freeboard plus 50 mm. For other ships, the Winter North Atlantic Freeboard shall be the winter freeboard.
Fresh-Water (F and TF)
The regulations apply to vessels at sea in salt water. An allowance is computed as a guide in compensating for deeper draft when a vessel loads in fresh water. The fresh water allowance in cm is determined by dividing the displacement at the summer load waterline by forty times the tons per cm immersion at the draft. The fresh-water allowance in inches is determined by dividing the summer load line displacement by forty times the tons per inch immersion at that draft.
If the basic information is not available, the allowance is taken at one forty-eighth of the summer draft. The fresh-water allowance applies to all seasonal freeboards, but fresh-water lines arc marked on the vessel only for the summer and tropical conditions. Where the water is brackish, a proportion of the fresh-water allowance is used, and a measurement of the actual density of the water at the place of loading may be required.
In addition to the fresh-water allowance, deeper loading is also permitted to allow for consumables used between the point of loading and the open sea.
If a Timber Freeboard is assigned it is marked in addition to the ordinary load lines with some slight variation. The Winter Timber Freeboard is obtained by adding to the Summer Timber Freeboard one thirty-sixth of the molded summer timber draft. The Winter North Atlantic Timber Freeboard is not to be less than the ordinary Winter North Atlantic Free¬board.
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